1. Field of the Invention
The present invention relates to voltage controlled oscillators suitable for use as a high-frequency signal oscillator and more particularly to an enhanced voltage controlled oscillator that has an increased variable range of oscillation frequency.
2. Description of the Related Art
Conventionally, a voltage controlled oscillator (VCO) has been available that is built in high-frequency semiconductor devices or the like. FIG. 1 is a circuit diagram showing a prior art voltage controlled oscillator.
In the prior art voltage controlled oscillator, an inductor L101 and an inductor L102 are each connected at one end thereof to a constant current power supply 101. The inductance of the inductor L101 is equal to that of the inductor L102. On the other hand, the inductor L101 is connected at the other end thereof with a varactor diode D101, the drain of a P-channel transistor Tr101, and the gate of a P-channel transistor Tr102, while the inductor L102 is connected at the other end thereof with a varactor diode D102, the drain of the P-channel transistor Tr102, and the gate of the P-channel transistor Tr101. An analog control voltage is applied to the varactor diodes D101 and D102, while a power supply voltage VDD is applied to the respective sources of the P-channel transistors Tr101 and Tr102.
In the prior art voltage controlled oscillator constructed as described above, it is possible to acquire an oscillation signal from the drains of the P-channel transistors Tr101 and Tr102. Assuming that the oscillation frequency of the oscillation signal is ω, the sum of the inductance of the inductors L101 and L102 is L, and the total sum of the capacitance and parasitic capacitance of the varactor diodes D101 and D101 is C. Then, the oscillation frequency ω is given by Equation (1) as follows. That is,ω=(LC)−1/2   (1)
In general, the varactor diodes D101 and D101 each comprise a MOS transistor as a basic component, with the total capacitance derived from the sum of the capacitance of a gate oxide film and a depletion layer present in the substrate. Accordingly, varying the bias applied to the gate of the MOS transistor that constitutes the varactor diode will cause the depletion layer to vary in depth, thereby resulting in a change in the total capacitance. That is, varying the control voltage will cause a change in the total capacitance C, thereby enabling the oscillation frequency ω to vary.
However, the prior art voltage controlled oscillator as described above has a variable range of oscillation frequency of about +/−15% with respect to its average frequency. There is thus a problem that no tuning can be made beyond this range. Since the control voltage causes no change in the capacitance of the gate oxide film and the depletion layer undergoes only a limited variation in capacitance, this problem arises due to the varactor diode itself having a variable range of capacitance limited to some extent and the presence of parasitic capacitance being not varied by the control voltage. For high-speed operations of oscillators demanded in recent years, the inductors have been designed to have lower inductance in order to provide higher oscillation frequencies. This has led to a greater rate of change in inductance depending on the shape of the inductor as the inductance becomes lower. In other words, a slight difference between the shapes of the designed inductor and an actually fabricated one has a significant effect on the oscillation frequency. Under these circumstances, when the variable range of oscillation frequency is limited as narrow as mentioned above, a serious problem may not arise at oscillation frequencies with a relatively small difference between the shapes of the inductors but may occur to such an extent that the oscillation frequency cannot be provided as designed, making the oscillator unavailable for service. Accordingly, this leads to a significant decrease in fabrication yield of the oscillator.
Concerning the variable range of oscillation frequency, a circuit is disclosed in Japanese Patent Laid-Open Publication No. Hei 8-162331. In the circuit, an inductor is incorporated into a circuit device and then its inductance is made tunable by providing a looped wiring on the layer of a spiral wiring to coincide with the spiral wiring in a plan view, such that use is made of the mutual magnetic induction of the looped wiring to tune the inductance of the spiral wiring. However, since an electric current needs to be supplied all the time not only to the spiral wiring but also to the looped wiring, this circuit consumes a large current. Additionally, a parasitic capacitance is built in the looped wiring. Furthermore, the mutual magnetic induction does not provide a large variable range of inductance, thereby resulting in an insufficiently increased variable range of oscillation frequency.